Refiners such as those discussed above are used for refining fibrous material. These refiners normally comprise refining members in the form of disks which rotate in relation to each other and between which refining material passes from the inner periphery of the refining members, where the refining material is supplied, to the outer periphery of the refining members, through a refining gap formed between the refining members. Often one of the refining disks is stationary while the other one rotates. The refining disks are generally constructed from refining segments provided with bars. The inner segments generally have a coarse pattern and the outer segments generally have a finer pattern in order to achieve fine refining of the refining material.
To ensure good quality refining material when refining fibrous material, the disturbances in operating conditions that continually occur for various reasons are corrected by continuous control of the various refining parameters to optimal values. This can be achieved, for instance, by altering the supply of water to give greater or less cooling effect, by changing the flow of refining material, or by adjusting the distance between the refining members, or a combination of these measures. To enable the necessary adjustments and corrections careful determination of the energy transmitted to the refining material is necessary, as well as the distribution of the energy transmitted across the surface of the refining members.
In order to determine the energy/power transmitted to the refining material it is known to attempt to measure the shearing forces occurring in the refining zone. It is known that a shearing force occurs when two surfaces move in relation to each other with a viscous liquid between them. Such a shearing force is also created in a refiner when refining wood chips are mixed with water. It can be imagined that the wood chips are both sheared and rolled between the refining disks, and also that collisions occur between wood chips and bars. The shearing force is dependent, inter alia, on the force of the disks as they are brought together as well as the friction coefficient. Furthermore, the normal force exerted on the surface varies with the radius.
A method and a measuring device are known for measuring stress forces in such refiners, as shown in International Application No. WO 00/78458, comprising a force sensor that measures the stress forces over a measuring surface constituting a part of a refining disk, and where the measuring surface comprises at least parts of more than one bar and is resiliently arranged in the surface of the refining disk. However, this measuring device has proved to be very sensitive to temperature variations, which are common in the relevant conditions, and it therefore often gives incorrect values for the stress, which cannot be used to control the refining process.
One object of the present invention is to solve the problems mentioned above and to thus provide a method and a measuring device that provides a more reliable result than known devices.